✋🏼 Hold up. A normal Read Speed but a terrible Write Speed can happen when you fail to enable TRIMforce when installing an SSD. IT is NOT engaged on our SSD. Happened to me, and it's fixable at home without disassembly
CAVEAT: Kingston is not one of the preferred SSDs for this purpose. Most senior contributors here that have done the conversion recommend only OWC Electras or Crucial's MX series (not the BX series) SATA SSDs. These steps may not play well with a Kingston.
1) First, read this article on how to engage TRIMforce, and make it so.
https://eshop.macsales.com/blog/31619-how-to-execute-trimforce-command-with-your-owc-ssd/
2) Once engaged, you still have work to do. Restart the computer in Safe Mode:
How to use safe mode on your Mac - Apple Support
Once a Safe boot is achieved, DO NOTHING! Walk away and let Safe Mode do whatever housekeeping and reindexing it needs. After 2-3 hours (yes), restart the computer normally and let it sit for another 30 minutes or so for Spotlight to do its thing.
3) Run another Etrecheck test to see is the Write Speeds improve. They should the in the 500±50 MB/sec range if Safe Mode completed its work.
4) If no change, repeat the process.
I had to safe boot several times over three days to regain speeds on my OWC Electra SSD which I originally installed without engaging TRIM.
Again I know this works with Crucial and OWC drives but do not have full confidence in Kingston for a number of personal reasons. If your cannot enable TRIM, the drive will have to be replaced anyway or your will need BDAqua's elegant but more expensive solution. To me it is worth the risk but it is ultimately yours to do risk assessment. It si not useful as is.
Thanks to Grant Bennet-Alder for his excellent help when my SSD did what yours is showing.
